1. Field of the Invention
The present invention relates to a waste gas exhaust system for use with apparatus for vacuum treating substrates with gases of the chlorine series, and more particularly to a waste gas exhaust system which is designed for long service life and easy maintenance.
2. Description of the Prior Art
In the manufacture of semiconductor integrated circuits, vacuum processes such as dry etching, plasma CVD, low-pressure CVD and so forth, which employ halides, will come into wider use. In ordinary vacuum process of this kind, a reactive gas of a halide or the like is introduced into a reaction chamber, wherein a reactive plasma is generated under lowered pressure for treating substrates in a vacuum. Consequently, the waste gas exhaust system for such a vacuum process apparatus degenerates in performance more seriously than do exhaust systems for other vacuum equipment. When a conventional exhaust system is used with the vacuum process apparatus employing halides, its exhausting capacity may sometimes be impaired in a day. Especially, in the case of dry etching apparatus for aluminum, which uses chlorinated compounds, such as CCl.sub.4, BCl.sub.3, Cl.sub.2, etc., since the exhaust gas contains not only unreacted gases but also strongly acidic reaction products, such as HCl and AlCl.sub.3, the exhaust system for discharging these reaction products undergoes more serious deterioration. Some solutions to this problem have been proposed so far, but any satisfactory exhaust system has not as yet been available which withstands severe conditions imposed during operation of vacuum process apparatus on a mass-production scale.
Conventionally, an exhaust system for a reactive dry etching apparatus includes valves, an oil-sealed rotary pump, a booster pump, a gas trap, pipes, an exhaust port and so forth. When aluminum is etched using a gas consisting principally of BCl.sub.3 gas, BCl.sub.3 and AlCl.sub.3 are adsorbed in large quantities in the gas trap to decrease its adsorbing capacity, resulting in lowering of the exhaust rate. Purging or cleaning of the gas trap to avoid it, however, poses a problem as it generates large quantities of HCl and B.sub.2 O.sub.3 powder, which clogs or corrodes the gas trap, a pipe for purging and the valves.
On the other hand, in the case in which no gas trap is provided, BCl.sub.3, HCl, AlCl.sub.3 and so on get mixed into oil of the oil-sealed rotary pump to degrade it and, further, the BCl.sub.3 reacts with the air in the oil-sealed rotary pump to evolve large quantities of B.sub.2 O.sub.3 power and HCl. The B.sub.2 O.sub.3 powder cannot be sufficiently filtered out by an oil filtration system, and shortens the service life of the pump.
A proposal has been made to fill the oil-sealed rotary pump with a inert gas. With this method, however, large quantities of B.sub.2 O.sub.3 and HCl are produced at the exhaust port; in particular, the B.sub.2 O.sub.3 powder blocks an exhaust duct to lower the ability of the oil-sealed rotary pump.
Further, AlCl.sub.3 is formed in large quantities, in addition to the B.sub.2 O.sub.3 powder, and it adheres to pipes, valves, etc. on the downstream side of the booster pump. If the pipes, valves and so on are heated for removing it, the AlCl.sub.3 powder reacts with water contained in the oil of the oil-sealed rotary pump to form strong acids, or it gets mixed into the oil, decreasing the efficiency of the oil-sealed rotary pump.